Conventional communication spacecraft or satellites are required to operate in a near-zero momentum condition, maintaining one body axis of the spacecraft, and the associated antennas, directed toward a particular location on the surface of the Earth. Zero-momentum spacecraft use torquers and control systems to orient the body of the spacecraft, and to maintain the desired body orientation.
The earliest spacecraft used body rotation to maintain attitude by virtue of angular momentum. This, in turn, required that, if an Earth-pointing instrument or antenna was required, that a portion of the body of the spacecraft be "de-spun". Later spacecraft eliminated the de-spinning function, by using torquers to control the attitude of the body, which in turn results in the instruments and/or antennas being directed in the desired direction.
In order to reduce the amount of propellant used by thrusters, zero-momentum spacecraft have in the past used electrically driven torquing arrangements. One such electrically driven torquing arrangement is in the form of magnetic coils mounted on the spacecraft body, which interact with the Earth's magnetic fields to produce body torque about the spacecraft roll and yaw axes. Because the North-to-South direction of the Earth's magnetic field lies along the pitch axis of a spacecraft in an equatorial orbit, magnetic torquing cannot be applied about the pitch axis.
Another form of electrically driven torquing arrangement includes momentum wheels or reaction wheels for providing torques about the control axes, and, in the case of momentum wheels, for providing gyroscopic stiffness about one or more of the control axes. These electrically-driven magnetic or wheel-type torquing arrangements eliminate the need to fire propellant-consuming thrusters for attitude control, which is very advantageous, because the available propellant can then be used exclusively for north-south and/or east-west stationkeeping, which results in a longer operational lifetime.
It has been found, however, that the momentum of the reaction or momentum wheels tends to increase with time in a secular manner. Because there is a limit on the rotational speed which a wheel can withstand, some additional torquing must be periodically applied to the spacecraft body, to maintain the momentum wheel rotational rate within limits. Magnetic torquers are used for reducing the wheel speed, and chemical thrusters are used when the magnetic torquers cannot provide sufficient torque. Thus, notwithstanding the presence of the two types of electrically-driven torquers on a spacecraft, it is still necessary to occasionally use chemical thrusters to provide torque, and the maximum remaining lifetime of the spacecraft is reduced as propellant is consumed.
The ANIK E2 spacecraft, launched in 1991 with an expected life of more than thirteen years, included magnetic torquers, a momentum wheel with a rotational axis lying along the pitch axis, pivotable about the roll axis, and also included a complement of chemical thrusters. Its attitude control system was autonomous, utilizing an Earth sensor assembly (ESA) for roll and pitch attitude sensing, together with pitch control torquing by wheel-speed variation. The yaw was not sensed; the yaw control was by gyroscopic stiffness in the roll and yaw axes, together with roll-yaw orbital interchange. Yaw/roll control was provided using magnetic torquers. As a result of an unusual anomaly in the space environment, both the primary and backup momentum wheels became inoperative, with the result that gyroscopic stiffness about the roll and yaw axes, and momentum-wheel pitch-axis attitude control were not possible. Since spacecraft are extremely expensive, it became very desirable to have some means for providing attitude control for a period of time commensurate with the remaining expected lifetime of the spacecraft.
Attitude control for the ANIK E2 after loss of the pitch wheels was complicated by the lack of spacecraft gyroscopic stiffness, and the loss of normal pitch control actuation. Because of the decoupling of the roll and yaw due to the lack of gyroscopic stiffness, a yaw measurement was needed, which had not originally been required, nor planned for. The available resources aboard the ANIK E2 included the magnetic torquers, the pitch and roll on-board ESA attitude sensor, and individual thruster actuators. A set of gyroscopes was also available, but these gyros had a design life of substantially less than the expected spacecraft life; consequently, the gyros could not be used continuously for attitude sensing for the desired duration of the mission. The on-board attitude processor was fully functional, but the autonomous attitude control processing logic portion which depended upon a momentum bias in its implementation was not reprogrammable. The two-way time lag for communications between an Earth or ground station and the spacecraft is about 0.25 second. The spacecraft uplink control channel was originally intended for use with an autonomous attitude control system, for commanding the operating state of communication transponders, and the like. The uplink channel has a two second command cycle. An attitude control system usable under the described conditions was needed.